1. Field of the Invention
The present invention relates to a Chemical-Mechanical Polishing (CMP) machine for polishing semiconductor wafers and, more particularly, to a wafer adapter for adapting a wafer in such a polishing machine.
2. Description of Related Art
CMP is a technology in the process for global planarization of a semiconductor wafer for VLSI (very large scaled integrated circuit) or ULSI (ultra large scaled integrated circuit). It utilizes a mechanical polishing method with a chemical reagent for polishing the surface of the wafer to provide planarity for the outer surface.
Referring to FIG. 1 and FIG. 2, a plane view and a sectional view of a conventional CMP machine are shown, respectively. It includes a polishing table 12 and at least one carrier 14 located upon the table for holding a wafer 10 which will be polished. The carrier 14 further includes a holder 16 and an adapter 24 for grasping the back of a wafer 10 and pressing the exposed surface of the wafer on a polishing pad 18 fixed on the polishing table 12. During the polishing process, the table 12 and the carrier 14 rotate in the opposite directions. There is also a polishing slurry 20 fed via a tube 22 to the table 12. Therefore, the exposed surface of the wafer 10 will be planarized after the polishing process.
Further, referring to FIG. 3, 4, 5 and 6, the configurations of two wafer adapters of prior arts are shown. In FIG. 3 and FIG. 4 (a sectional view taken from line B--B of FIG. 3), the adapter 24 includes a retaining ring 32 with a inner circle for holding a wafer; an unshown fastener such as a screw for fastening the retaining ring 32 to a holder; and a carrier film 34 laid between the ring 32 and the holder 16 to cushion the wafer. Generally, a CMP machine is designed for a specific dimension of wafer. Taking an example of a wafer of eight inch diameter, the adapter 24 for the holder is originally designed in which the inner diameter of the retaining ring 32 is just for grasping such a wafer, and the width T of the ring 32 is just of a few millimeters. The width is so designed in order not to block the transportation of slurry from the polishing pad to the wafer. When a CMP machine of eight inch is required to polish a six inch wafer, a conventional adapter 24 as shown in FIG. 5 and FIG. 6 (a sectional view taken from line C--C of FIG. 5) is used. Since the outer diameter of the retaining ring 32 is restricted to the configuration of the holder, only the inner diameter of the ring is modified. As the inner diameter of the ring 32 is shrank to six inches, there comes a two-inch wide barrier for the slurry transportation. Though there are several straight grooves 36 cut for an easier transportation of the slurry, there are some disadvantages for the conventional adapter:
a) the straight grooves 36 make the slurry flow easily between the wafer and the polishing pad but cannot efficiently hold the slurry during the polishing process, increasing the need of more slurry and decreasing the efficiency of slurry utilization; PA1 b) the straight grooves 36 are ineffective for the slurry distribution and sensitive to the process parameters such as the table rotation speed and the carrier rotation speed; PA1 c) there are sharp comers formed on the wall of the inner circle of the ring due to the grooves 36, which are in contact with the wafer. If the wafer rotates inside the ring during the polishing process, it is possible to have collision between the wafer and these sharp corners, resulting in wafer loss; PA1 d) the straight grooves 36 breaking through the surface of the inner circle of the retaining ring 32 reduce the contact area of the ring to the wafer, therefore reducing the lifetime of the ring due to the smaller contact area and higher contact pressure.